Let me point you to a nice article, by Simon Fisher and Gary Marcus, in the January issue of Nature Reviews Genetics. Fisher was one of the co-discovers of the link between a verbal dyspraxia disorder in an English family and a point mutation on the FOXP2 gene. Marcus, a former student of Steven Pinker, is well-known for his critique of connectionist models of language-aquisition. Although I personally think it overstates the case for innate modules, I wholeheartedly recommend Marcus' book The Birth of the Mind which is, as far as I know, the first, and still only, popular account of what is known today about the genetic basis of brain development. Together, they have earlier written a review of the FOXP2 story in Trends in Cognitive Science. The present paper aims to show how genetical studies can aid our understading of the evolution of language.
In recent years interest in this topic has been booming. A large number of possible adaptative causes have been proposed by a range of researchers. [Language Evolution, edited by Morten Christiansen and Simon Kirby, will give you a short overview of most of these hypotheses.] There is a problem with this approach, though, in that it tends to focus on just one magical change in hominid behaviour, with the accompaying change to the brain being passed on to following generations. To take one prominent example, Robin Dunbar, for instance, sees the decisive moment in the evolution of language to be the expansion of hominid social groups, some time around the appearance of homo erectus, making it advantageous to be able to keep account of the larger number of conspecifics through communication. Dunbar speculates that a change in our ability to mentalize – sometimes also referred to as Theory of Mind – could be the new cognitive function to have facilitated this improved capacity for social communication. Another, more simpleminded hypothesis, is the still widespread idea that neuronal changes to Broca's area brought about a novel ability to string together words into syntactical phrases.
But, as more and more becomes known about the brain processes underlying language, it appears increasingly unrealistic that any such single-stroke wave of the magic wand will suffice to explain the emergence of language. The neural language system is enormously complex and encompasses, to just name a few things, auditory analysis, conceptual knowledge and memory, semantic selection processes, motor control, and a great number of other functions. It stands to reason that all these processes must have evolved on an individual basis. It is therefore much more probable that the evolution of language has gone through several different adaptive events since the last common forefather of homo and pan.
This is exactly the approach taken by Fisher and Marcus. They call it "descent with modification", and write that "[in this paper] we argue that language should be viewed not as a wholesale innovation, but as a complex reconfiguration of ancestral system that have been adapted in evolutionarily novel ways". The offshot of this approach is that the evolution of language can be studied by comparing human genetics and neurobiology to other species. Write Fisher and Marcus:
(…) although non-human primate communication shows qualitative differences from human language, studies have established that most components of language how some degree of continuity with other species. For example, the human vocal tract supports a wider epertoire of speech sounds than could be produced by other primates, but the capacity to create richly modulated formants is not unique to humans. Likewise, many animals and birds can distinguish different human speech sounds, and adult tamarin monkeys can discriminate between the distinctive rhythmic properties of different languages. Debate continues about exactly how much of the machinery of language is species – or language – specific; for example, opinion is divided over whether recursion represents the only component that is genuinely new to the human species. Nevertheless, views that consider language to be fully independent of ancestral systems are no longer tenable, and there is a growing recognition that cognitive, physiological, neuroanatomical and genetic data from non-speaking species can greatly inform our understanding of the nature and evolution of language.
The bulk of Fisher and Marcus' paper is dedicated to a review of methods for such genetical comparisons of humans to other species. But its true importance lies in the way it dismantles the adaptionist programme in evolutionary psychology. Like language, social cognition, reasoning, and other forms of cognition, are in most cases complex amalgams of a range of neurobiological processes, and therefore very unlikely the product of "wholesale innovation". A more precise picture of the evolution of human cognition is to be found through an detailed understanding of how the human brain differs from other species, and through a comparison of the genes and gene-expression systems characterising these species.
Fiser, S. & Marcus, G. (2006): The eloquent ape: genes, brains and the evolution of language. Nature Reviews Genetics 7: 9-20. [The paper can be downloaded from Marcus's homepage.]